This invention is concerned with the electrodeposition of bright chromium on basis metals from hexavalent chromium plating baths at high current efficiencies.
In the past, ordinary hexavalent chromium plating baths containing chromic acid and a catalyst such as sulfate ion generally permit the deposit of chromium metal on the basis metal at cathode efficiencies of between 12% and 16% at temperatures between about 125.degree. F. and 155.degree. F. (52.degree. C. to 68.degree. C.) and at current densities of from about 30 to about 50 a.s.d. Mixed catalyst chromic acid plating baths containing both sulfate and fluoride ions generally allow the plating of chromium at higher rates and at cathode efficiencies of between 22% and 26%. Fluoride ion however, causes etching of ferrous based metals when the cathode current density is too low to deposit chromium metal, usually below about 5 a.s.d. in fluoride containing baths. This phenomenon is called low current density etch.
Generally, the properties of a chromium deposit vary with certain principal deposition factors, particularly temperature and current density. Useful deposits are associated with the bright or semi-bright range. In an ordinary sulfate-catalyzed bath at 30.degree. C., bright deposits are obtained from about 2 a.s.d. to 8 a.s.d.; at 40.degree. C. they are obtained from about 3 a.s.d. to 18 a.s.d. and at 50.degree. C., from about 6 a.s.d. to 28 a.s.d. (Ref.: Chromium Plating, R. Weiner & A. Walmsley, Finishing Publications Ltd., Teddington, Middlesex, England, 1980 page 52). Milky deposits are produced below the low current densities for each temperature, i.e. below 2 a.s.d. at 30.degree. C., 3 a.s.d. at 40.degree. C. and 6 a.s.d. at 50.degree. C., while frosty deposits are obtained above the higher current densities for each temperature, i.e. above 8 a.s.d. at 30.degree. C., 18 a.s.d. at 40.degree. C. and 28 a.s.d. at 50.degree. C. Abrasive wear resistance which is associated with hardness is at a maximum within the frosty bright region of the bright range. Corrosion resistance, another important property, is at a maximum in the milky region of the bright range. Bright deposits are achieved between the frosty and milky regions and are generally characterized by having intermediate abrasive wear resistance and corrosion resistance.
Chromium plating baths have been recently developed by Perakh et al (see U.S. Pat. No. 4,234,396, for example) which contain from 100 g to 1600 g chromium trioxide per liter and, based on the chromium trioxide content, 0.3 to 15 wt. percent chlorine or chloride ions and/or 0.3 to 10 wt. percent iodine and/or iodide ions. Perakh baths containing chlorine or chloride ions alone generally yield dull to semibright deposits, the semi-bright deposits occurring at low temperatures (19.degree. C.). When iodine or iodide ions are used alone in such baths, semi-bright deposits are still attained at low temperatures (&lt;24.degree. C.). In the case of Perakh baths containing both halogen species, bright deposits are achieved but only at bath temperatures not exceeding about 50.degree. C.
The present invention, on the other hand, provides a chromium plating bath containing additives which produce bright chromium deposits at current efficiencies of over 30%, more often 40-50%, over a wide range of current densities and with no low current density etch. Moroever, unlike the Perakh-type baths, bright deposits may be achieved at high temperatures (i.e. greater than 50.degree. C.). The high bath temperatures allow bright plating at wider ranges of current densities than at lower temperatures and also promote adherence of the deposit.